Conventionally, there has been widely known an air-fuel ratio control apparatus, which includes a three-way catalyst (43) disposed in an exhaust passage of an internal combustion engine, and an air-fuel ratio sensor (56) disposed upstream of the three-way catalyst (43), as shown in FIG. 1.
This air-fuel ratio control apparatus calculates an air-fuel ratio feedback amount (quantity) based on the output of the air-fuel ratio sensor (56) in such a manner that an air-fuel ratio (an air-fuel ratio of the engine, and thus, an air-fuel ratio of an exhaust gas) of an air-fuel mixture supplied to the engine coincides with a target air-fuel ratio, and feedback-controls the air-fuel ratio of the engine based on the air-fuel ratio feedback amount. The air-fuel ratio feedback amount used in such an air-fuel ratio control apparatus is a control amount commonly used for all of the cylinders. The target air-fuel ratio is set at a base (reference) air-fuel ratio which is within a window of the three way catalyst (43). The base air-fuel ratio is typically equal to a stoichiometric air-fuel ratio. The base air-fuel ratio may be changed to an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio base on an intake air amount of the engine, a deterioration degree of the three way catalyst (43), and so on.
Incidentally, in general, such an air-fuel ratio control apparatus is applied to an internal combustion engine using an electronic-control-fuel-injection apparatus. The internal combustion engine has at least one fuel injection valve (33) at each of cylinders or at each of intake ports communicating with the respective cylinders. Accordingly, when the characteristic/property of the fuel injection valve of a certain (specific) cylinder changes so as to inject fuel in an amount excessively larger than an injection amount to be injected according to an instruction (instructed fuel injection amount), only an air-fuel ratio of an air-fuel mixture supplied to that certain cylinder (the air-fuel ratio of the certain cylinder) greatly changes toward the rich side. That is, the degree of air-fuel ratio non-uniformity among the cylinders (inter-cylinder air-fuel ratio variation; inter-cylinder air-fuel ratio imbalance) increases. In other words, there arises an imbalance among “cylinder-by-cylinder air-fuel ratios”, each of which is the air-fuel ratio of the air-fuel mixture supplied to each of the cylinders.
It should be noted that, hereinafter, a cylinder corresponding to the fuel injection valve having the characteristic to inject the fuel in an amount excessively larger or excessively smaller than the instructed fuel injection amount is also referred to as an imbalanced cylinder, and each of the remaining cylinders (a cylinder corresponding to the fuel injection valve having the characteristic to inject the fuel in an amount equal to the instructed fuel injection amount) is also referred to as an un-imbalanced cylinder (or a normal cylinder).
When the characteristic/property of the fuel injection valve of the certain (specific) cylinder changes so as to inject fuel in the amount excessively larger than the instruction injection amount, an average of the air-fuel ratio of the air-fuel mixture supplied to the entire engine becomes richer than the target air-fuel ratio which is set at the base air-fuel ratio. Accordingly, by means of the air-fuel ratio feedback amount commonly used for all of the cylinders, the air-fuel ratio of the above-mentioned certain cylinder is changed toward the lean side so as to come closer to the base air-fuel ratio, and, at the same time, the air-fuel ratios of the remaining cylinders are changed toward the lean side so as to deviate more greatly from the base air-fuel ratio. As a result, the average (air-fuel ratio of the exhaust gas) of the air-fuel ratio of the air-fuel mixture supplied to the entire engine becomes equal to an air-fuel ratio in the vicinity of the base air-fuel ratio.
However, the air-fuel ratio of the certain cylinder is still in the rich side in relation to the base air-fuel ratio, and the air-fuel ratios of the remaining cylinders are in the lean side in relation to the base air-fuel ratio. Consequently, an amount of emissions (an amount of unburned combustibles(substances) and/or an amount of nitrogen oxides) discharged from each of the cylinders increase, as compared to the case in which each of the air-fuel ratios of the cylinders is equal to the base air-fuel ratio.
Therefore, even when the average of the air-fuel ratio of the mixture supplied to the engine is equal to the base air-fuel ratio, the increased emissions cannot be removed by the three-way catalyst. Consequently, the amount of emissions may increase.
Accordingly, in order to prevent the emissions from increasing, it is important to detect a state in which the degree of the non-uniformity among the cylinder-by-cylinder air-fuel ratios becomes excessively large (i.e., the non-uniformity of the air-fuel ratio among the cylinders becomes excessively large, that is, generation of an inter-cylinder air-fuel ratio imbalance state), and to take some measures against the imbalance state. It should be noted that, the inter-cylinder air-fuel ratio imbalance also occurs, for example, in a case where the characteristic of the fuel injection valve of a certain cylinder changes to inject fuel in an amount excessively smaller than the instructed fuel injection amount.
One of conventional fuel injection amount control apparatuses obtains a trace/trajectory length of the output value (output signal) of the upstream air-fuel ratio sensor (56). Further, the control apparatus compares the trace length with a “reference value which changes in accordance with an engine rotational speed”, and determines whether or not the inter-cylinder air-fuel ratio imbalance state has occurred based on the result of the comparison (see, for example, patent literature No. 1).
Meanwhile, when the non-uniformity among the cylinder-by-cylinder air-fuel ratios occurs, there may be a case in which a true average of the air-fuel ratio of the engine is controlled so as to become an air-fuel ratio larger than the base air-fuel ratio (leaner than the base air-fuel ratio) by means of the feedback control (main feedback control) to have an air-fuel ratio represented by the output value of the air-fuel ratio sensor (56) coincide with the “target air-fuel ratio which is set at the base air-fuel ratio such as the stoichiometric air-fuel ratio.” The reason for this will next be described.
The fuel supplied to the engine is a chemical compound of carbon and hydrogen. Accordingly, the unburnt substances such as “carbon hydride HC, carbon monoxide CO, and hydrogen H2” are generated as intermediate products, when the air-fuel ratio of the mixture to be combusted is richer than the stoichiometric air-fuel ratio. In this case, as the air-fuel ratio of the mixture for the combustion becomes richer in relation to the stoichiometric air-fuel ratio and deviates more greatly from the stoichiometric air-fuel ratio, a probability that the intermediate products meet and bind to the oxygen molecules during the combustion becomes drastically smaller. Consequently, as shown in FIG. 2, an amount of the unburnt substances (HC, CO, and H2) drastically (e.g., in a quadratic function fashion) increases, as the air-fuel ratio of the mixture supplied to the cylinder becomes richer.
It is now assumed that a non-uniformity among the cylinder-by-cylinder air-fuel ratios occurs where only the air-fuel ratio of a certain cylinder greatly deviates toward the rich side. Under this assumption, the air-fuel ratio (air-fuel ratio of the certain cylinder) of the air-fuel mixture supplied to that certain cylinder changes to a much richer (smaller) air-fuel ratio, compared to the air-fuel ratios (air-fuel ratios of the remaining cylinders) of the air-fuel mixtures supplied to the remaining cylinders. At this time, a great amount of unburnt substances (HC, CO, and H2) are discharged from that certain cylinder. Accordingly, even when the average of the air-fuel ratio of the mixtures supplied to the engine coincides with a “certain (specific) air-fuel ratio”, a total amount of hydrogen discharged from the engine when the degree of the non-uniformity among the cylinder-by-cylinder air-fuel ratios becomes large is significantly larger than a total amount of hydrogen discharged from the engine when the non-uniformity among the cylinder-by-cylinder air-fuel ratios is not occurring.
In the mean time, the air-fuel ratio sensor (56) comprises a porous layer (e.g., a diffusion resistance layer, or a protective layer) that makes a “gas (gas after oxygen equilibrium) which is in a state where the unburnt substances and oxygen have chemically achieved equilibrium” reach the air-fuel ratio detection element. The air-fuel ratio sensor (56) outputs a value corresponding to “an amount of oxygen (oxygen partial pressure, oxygen concentration) or an amount of unburnt substance (unburnt substance partial pressure, unburnt substance concentration)” that has reached an exhaust-gas-side electrode layer (surface of the air-fuel ratio detection element) of the air-fuel ratio sensor (56) after passing through the diffusion resistance layer.
Meanwhile, hydrogen H2 is a small molecule, compared with carbon hydride HC, carbon monoxide CO, and the like. Accordingly, hydrogen H2 rapidly diffuses through the porous layer of the air-fuel ratio sensor (56), compared to the other unburnt substances (HC, CO). That is, a preferential diffusion of hydrogen H2 occurs in the porous layer.
Due to the preferential diffusion of hydrogen when the non-uniformity among the cylinder-by-cylinder air-fuel ratios (air-fuel ratio imbalance among the cylinders) is occurring, the output value of the air-fuel ratio sensor (56) shifts to a value in a richer side. Thus, the air-fuel ratio represented by the output value of the air-fuel ratio sensor (56) becomes an “air-fuel ratio in the richer side” with respect to a true air-fuel ratio of the engine. Consequently, due to the main feedback control, the average of the air-fuel ratio of the engine is controlled so as to be the “air-fuel ratio larger than (leaner than) the base fuel air-fuel ratio.”
In contrast, an exhaust gas which has passed through the three-way catalyst (43) arrives at an air-fuel ratio sensor (57) disposed at a position downstream of the three-way catalyst (43). The hydrogen is purified to some degree by the three-way catalyst (43). Accordingly, the output value of the downstream air-fuel ratio sensor is in the vicinity of an output value corresponding to a true average of the air-fuel ratio of the engine, even when the degree of the non-uniformity among the cylinder-by-cylinder air-fuel ratios becomes large.
In view of the above, another conventional apparatus is configured so as to determines whether or not the degree of the non-uniformity among the cylinder-by-cylinder air-fuel ratios becomes large, based on a parameter indicative of a difference between the air-fuel ratio represented by the upstream air-fuel ratio sensor (56) and the air-fuel ratio represented by the downstream air-fuel ratio sensor (57) (see, for example, patent literature No. 2).